[SBE] Ultrasonic Leak Detection

[Chris Spacone]

Edwin,

The short answers are no, I haven't disassembled the system and no I haven't
listened to the lines. Listening to the lines with a stethoscope sounds like
an excellent idea and one that I had not even considered (ahhh, the wisdom
of the mailing list prevails!). I will get a stethoscope and do so first
thing on Monday morning. Hopefully it will be noisy enough to be heard over
the background noise.

A recap may be in order:

I was initially dealing with the pressurization manifold. It consisted of a
compressed air supply from the house compressor that fed a regulator check
valve (limited to about 10 psi), a desiccant canister (which still needs to
be serviced), a low pressure regulator set to about 3 psi, then into a 3 leg
manifold. Each leg has a ball valve and pressure gauge that feeds the
associated waveguide via polyethylene tubing and compression fittings.
Because of construction in the building I had to remove the house air and
the DirEng said get N2 and install it ASAP. I complied.

I found 3 major leaks on the manifold assembly described above. One was at
the house air to manifold interface. The second was a poorly assembled poly
line / compression nut (it was actually folded over!) and finally a 1/4"
brass 'Tee'. The brass tee was the hardest to find because it had the most
obvious leak! After much hunting I was exasperated enough to start wrapping
all of the fittings in a soaking, soapy paper towel. Voila! Found the tee
was actually cracked down the back. It was a cheap fitting, cast then
machined and over tightened by the original installer (probably in a fit of
rage while trying to get rid of the leaks).

All those problems are gone now but the system still doesn't hold air (or in
this case N2) so clearly the problem lies downstream of the manifold.
Initially I started this thread because of the location of the elliptical
waveguide flanges and pressure windows: directly over the top of the HPA's.
Clearly, I am not putting soapy water anywhere near the HPA's (yes guys, I
realize I can do this safely, but I'd rather not tempt the Fates). So I
looked into leak detection. In a previous life I worked at a medical
instrumentation startup and used vacuum leak detection utilizing helium as a
tracer gas. I'm not about to invest thousands on a bad idea: pulling a
vacuum on waveguide is the antitheses of what I am trying to accomplish. I
have no intention of pulling beach air (I'm in LA about 2 miles from the
shore), laden with moisture and NaCl, into the waveguide.

So, keeping the pressure applied (and admittedly delivering an unknown
amount of N2 back into the atmosphere) I decided to look for alternatives.
Being a HAM (KD6OUB) I had read a fairly recent article in QST regarding
using ultrasonic's to detect arcing on power poles in an attempt to find and
mitigate HF interference. I figured this method could be used to find gas
leaks. I did a quick search on the internet (Google is your friend!) turned
up a bunch of hits. That's when I decided to drop a note to the list and see
if anybody had experience with this leak detection method. Clearly, the
thread has wandered a bit but I'm taking away the conclusion that the method
is probably viable but may or may not be effective in my installation. In
any case I'll be asking the boss to buy one of the smaller, less expensive
(and probably low capability) units before I start shotgunning the problem.
This is our backup uplink and supports our ad hoc HD feeds so is considered
'revenue generating'.

I'm not about to do anything that would jeopardize that backup facility or
revenue stream. If it becomes an emergency then it becomes another story.
I'm not suggesting that you (or anybody else on the list) advocates
shotgunning, just stating my reluctance to do so.

One point you make about ease of access is the unfortunate fact that the
three sections of elliptical exit the building and go underground thru
conduit to the dishes. God, please don't let it be in there!

73,
Chris

-----Original Message-----
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of Edwin
Bukont
Sent: Saturday, May 17, 2008 9:39 AM
To: sbe member discussion mail list
Subject: Re: [SBE] Ultrasonic Leak Detection


J. Fred Reily always said 'its not the tube' and the leak is not always on
the tower.
 
Several engineers and tower folks had spent the better part of 15-20 years
dealing with a leak on 600' of 3-1/8 rigid line.  By the time I got there,
the place was being pressurized with a dewar...the equivalent of I think 8
typical N2 tanks.  I did replace that with a dehydrator, which had to be
serviced after about a year.  New station owners said to have the tower crew
inspect the line again (mind you, it was always the same tower crew) when
doing the due diligence.
 
In the process of their inspection, for whatever reason, I stepped up on the
base pier of the tower and wrapped my arm around the transmission line,
which was now about ear level....and thought I heard something rushing in
the line.   A return visit with stethoscope indicated there was indeed some
sort of noise in the line.  Working back along the  line, it got louder as I
went towards the building.  Since the level got louder, it was unlikely to
be air rushing in the line, but maybe something vibrating the line.
 
It turned out to be where the line entered the building, and some knuckle
head had mounted the coax switch almost flat to the outside wall (straight
thru, not with elbows and into the switch, right above the dummy load, with
just barely enough room for the downward transition to the dummy load.  They
used a short peice of solder flange line to get from the switch to outside,
such that the flanges were inside the block wall.  The wall was sealed, of
course.  The rigid line seal was damaged, probably from wrestling with the
too tight fit.  The noise of the gas was not noticed inside the building
because the building is noisy and the gas was venting into the wall.  All
that wasted time and cost to inspect a leak that could have been avoided by
proper, rather than cute, engineering.  The building was big enough and had
enough ceiling clearance to properly mount the switch and bring a proper run
of line inside the room.  
 
The point being,,,has the system been fully disassembled  at the easiest to
access end and fully inspected rather than assuming the leak is further
along.  Have you tried listening for any noise in the line?
Edwin Bukont CSRE, DRB, CBNT Comm-Struction and Services LLC P.O. Box 629;
Bel Air, MD 21014 USA V- 410.879.5567 F- 240.368.1265 C- 240.417.2475
ebukont at msn.com Member: IEEE, SBE, AES, PMI Digital Media and Power Systems
Integrators. A Harris Broadcast Channel Partner 
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